Metal powder sintering paste and method of producing the same, and method of producing conductive material

ABSTRACT

There is a problem that when a silver powder sintering paste that is substantially free from resin is used, an organic solvent used as a dispersion medium bleeds, which results in contamination and wire bonding defects. In order to solve the problem, provided is a metal powder sintering paste that contains, as a principal component, silver particles having an average particle diameter (a median diameter) of 0.3 μm to 5 μm and further contains an anionic surfactant but is substantially free from resin.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a 37 C.F.R. § 1.53(b) continuation of U.S.application Ser. No. 15/477,868 filed Apr. 3, 2017, which claimspriority on Japanese Patent Application No. 2016-075314 filed Apr. 4,2016. The entire contents of each application is hereby incorporated byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present disclosure relates to a metal powder sintering paste, amethod of producing the same, and a method of producing a conductivematerial.

2. Description of Related Art

Conventionally, in a power semiconductor device or a light-emittingdevice that has a light emitting element as a light source, it has beenknown that a metal powder sintering paste in which metal particles aredispersed in a dispersion medium such as an organic solvent is usedwhen, for example, a power semiconductor element is placed on a mountingmember. A metal powder sintering paste is placed between a mountingmember and, for example, a power semiconductor element, and heating themat around 200° C. allows the metal particles in the metal powdersintering paste to sinter to one another and thereby they are joinedtogether.

Furthermore, as a method of joining a light emitting element and amounting member together, a method has been known in which an adhesivecontaining a resin or a lead-free solder containing eutectic crystalsused therein is placed between a light emitting element and a mountingmember.

However, since the lead-free solder generally has a melting point of atleast 300° C., the member might be damaged due to member deteriorationcaused by heating at a high temperature or stress caused by thedifference in coefficient of thermal expansion between a substrate andthe member at the time of cooling after they are joined. Therefore, inthe method using a lead-free solder, the options of usable members cansometimes be limited.

In a metal powder sintering paste, when, for example, silver is used asthe metal powder, although silver has a melting point of approximately962° C., which becomes the theoretical heat resistance limit, joining iscarried out by heating at around 200° C. Therefore, the metal powdersintering paste is more advantageous in terms of joining temperature andheat resistance of the light-emitting device to be obtained, as comparedto the lead-free eutectic crystal solder containing eutectic crystalsused therein.

Furthermore, in a metal powder sintering paste, when gold is notcontained as the metal powder, it is less expensive as compared to agold-tin eutectic crystal solder that is often used for mounting a lightemitting element.

There are some metal powder sintering pastes that contain resins.However, from the perspectives that sintering requires a hightemperature and a resin component or the volatile portion of the resincontaminates peripheral members, the better ones are those that are freefrom resin and contain a volatile organic solvent as a dispersionmedium.

As compared to a method using an adhesive containing resin, a methodusing a metal powder sintering paste is excellent in terms of the heatresistance and heat dissipation of a light-emitting device obtainedthereby since resin degradation does not occur.

A method of producing a metal member joined body using a material forjoining a metal member as a metal powder sintering paste is known,wherein the material for joining a metal member is, for example, in theform of a paste composed of heat sinterable metal particles and a fluxand becomes a porous sintered material having a melting point equivalentto that of said metal particles when being heated to be sintered (forexample, JP5301385B). Specifically, this method is a method of producinga metal member joined body, wherein the method includes interposing apaste-like material composed of silver particles or copper particles (A)and a liquid material (B) between a plurality of metal members, heatingthem at a temperature between 200° C. and 400° C. to sinter the silverparticles or copper particles (A) to one another to become a poroussintered material, and thereby joining the plurality of metal members toeach other, the silver particles or copper particles (A) have an averageparticle diameter (a median diameter D50) of 0.2 μm to 10 μm and amelting point higher than 400° C. and have heat sinterability, and theliquid material (B) is composed of (b) an oxide film removal activator,namely, hydrochloride salt or bromate salt of amines, carboxylic acid,or organic halide, (c) a thixotropic agent, and (d) a solvent, or iscomposed of (b) an oxide film removal activator, namely, hydrochloridesalt or bromate salt of amines, carboxylic acid, or organic halide, and(d) a solvent, and the porous sintered material has a melting pointequivalent to that of the silver particles or copper particles (A) and aporosity at cross-section of 5 to 50% by area.

Furthermore, as a metal powder sintering paste, a metal particulatedispersion also has been known in which metal particulates with adispersant adsorbing on the surfaces thereof are dispersed in adispersion medium, said dispersant having at least one of a carboxylgroup and a hydroxy group in its molecule, and said dispersion mediumhaving a dielectric constant of at least 12.0 (JP2007-200775A).

Furthermore, as a metal powder sintering paste, a metal particulatedispersing liquid has been known in which metal particulates with thesurfaces thereof being coated with fatty acid and aliphatic amine aredispersed in a hydrophobic solvent containing a fatty acid derivativeadded thereto, the fatty acid derivative being fatty acid methyl esteror fatty acid ethyl ester having 12 to 20 carbon atoms (JP5778494B).

A volatile organic solvent that is used for metal powder sinteringpastes has a lower viscosity than that of an adhesive containing resin.Therefore, fine irregularities of the substrate-side electrode surfaceto be joined often cause an organic solvent component to wet/spread. Thewetting/spreading of the organic solvent component is a bleedingphenomenon caused by a capillary phenomenon. This causes bleeding in anunexpected portion in the substrate, which causes the organic solvent toleak onto the substrate bottom surface during the process or results inwire bonding defects due to contamination.

SUMMARY OF THE INVENTION

Therefore, the present disclosure provides a metal powder sinteringpaste that prevents bleeding from occurring, and a method of producingthe same, as well as a method of producing a conductive material usingthe metal powder sintering paste.

A metal powder sintering paste according to an embodiment of the presentdisclosure contains, as a principal component, silver particles havingan average particle diameter (a median diameter) of 0.3 μm to 5 μm andfurther contains an anionic surfactant but is substantially free fromresin.

A method of producing a metal powder sintering paste according to anembodiment of the present disclosure includes mixing an anionicsurfactant with silver particles having an average particle diameter (amedian diameter) of 0.3 μm to 5 μm, wherein substantially no resin ismixed together.

A method of producing a conductive material according to an embodimentof the present disclosure includes a step of calcining the metal powdersintering paste.

The metal powder sintering paste of the present disclosure can preventbleeding from occurring during heating.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, there are described a metal powder sintering paste and amethod of producing a metal powder sintering paste as well as aconductive material and a method of producing a conductive material ofthe present embodiments.

The metal powder sintering paste of the present disclosure contains, asa principal component, silver particles having an average particlediameter (a median diameter) of 0.3 μm to 5 μm and contains an anionicsurfactant but is substantially free from resin.

Furthermore, the method of producing a metal powder sintering paste ofthe present disclosure includes mixing an anionic surfactant with silverparticles having an average particle diameter (a median diameter) of 0.3μm to 5 μm, wherein substantially no resin is mixed together.

In the metal powder sintering paste of the present disclosure, theanionic surfactant exhibits the resistance to bleed provided by itselectric field with respect to an electrode such as silver or gold thatgenerally has a negative surface potential. This improves the leakageonto the substrate bottom surface and the wire bonding defects caused bycontamination, which makes it possible to produce reliable semiconductordevices, etc.

[Silver Particles]

In the metal particles to be used for the metal powder sintering paste,silver particles are the principal component. This means that thecontent of the silver particles contained in the metal particles is, forexample, at least 70% by mass, preferably at least 80% by mass, and morepreferably at least 90% by mass. The silver particles to be used hereinmay be one type of silver particles having the same average particlediameter (median diameter) or a mixture of at least two types of silverparticles that are different in average particle diameter. The silverparticles have an average particle diameter (a median diameter) of 0.3μm to 5 μm, preferably 1.0 μm to 4 μm, and more preferably 1.5 μm to 3.5μm. This can lower the electrical resistance value. Metal particles tobe used other than the silver particles can be those having an averageparticle diameter (a median diameter) of 0.1 μm to 15 μm, but theaverage particle diameter thereof is preferably 0.3 μm to 10 μm, morepreferably 0.3 μm to 5 μm.

With respect to the silver particles, the content of the particleshaving a particle diameter of less than 0.3 μm is preferably not morethan 5% by mass, more preferably not more than 4% by mass.

With respect to the silver particles, the content of the particleshaving a particle diameter of not more than 0.5 μm is preferably notmore than 15% by mass, more preferably not more than 10% by mass.

The average particle diameter (the median diameter) of the silverparticles can be measured by a laser diffraction method. The averageparticle diameter (the median diameter) denotes the value where theaccumulated frequency by volume is 50%, which is obtained from aparticle size distribution. Hereinafter, the average particle diameterdenotes the median diameter unless otherwise noted.

Furthermore, the silver particles have a specific surface area of 0.4m²/g to 1.5 m²/g, preferably 0.6 m²/g to 0.9 m²/g, and more preferably0.66 m²/g to 0.74 m²/g. This can increase the area where adjacent silverparticles are joined to each other, and since the addition of the silverparticles results in a small increase in viscosity, the paste cancontain many silver particles. This prevents the occurrence of voids andallows a high joining strength to be obtained. The specific surface areaof the metal particles that are the main raw materials of the metalpowder sintering paste can be measured by the BET method.

The shape of each silver particle is not limited. However, examplesthereof include a spherical shape, a flat shape, a flake shape, and apolyhedral shape, but the flake shape is preferred. This is because theflake shape increases the area of each silver particle where it is incontact with adjacent silver particles and decreases the electricresistance. The shapes of the metal particles each having an averageparticle diameter within a predetermined range are preferably uniform.When two or more types of metal particles that are different in averageparticle diameter are mixed, the shapes of the metal particles with therespective average particle diameters may be the same or different fromeach other. For example, when two types of metal particles, namely,those with an average particle diameter of 3 μm and those with anaverage particle diameter of 0.3 μm are mixed together, the metalparticles with an average particle diameter of 0.3 μm may have aspherical shape while the metal particles with an average particlediameter of 3 μm may have a flat shape.

The metal powder sintering paste contains silver particles as itsprincipal component. In the metal powder sintering paste, one type ormore of metal particles other than silver particles can be used andexamples thereof include gold, copper, platinum, palladium, rhodium,ruthenium, iridium, and osmium.

The content of the silver particles is preferably at least 70% by mass,more preferably at least 85% by mass, and further preferably at least90% by mass, with respect to the paste. This is because when the contentof the silver particles is within the predetermined ranges, the joiningstrength of the resultant conductive material increases.

[Surfactant]

The metal powder sintering paste contains an anionic surfactant. Due tothe electric field generated by the anionic nature, the anionicsurfactant exhibits the resistance to bleed provided by its electricfield with respect to an electrode such as silver or gold that generallyhas a negative surface potential.

This improves the leakage onto the substrate bottom surface and the wirebonding defects caused by contamination, which makes it possible toproduce reliable semiconductor devices.

The surfactant has preferably a high volatility. Specifically, when thetemperature is raised from around room temperature at 2° C./min inthermogravimetric-differential thermal analysis (TG-DTA), those in whichthe residue obtained at 350° C. is not more than 20% by mass withrespect to the initial mass are preferable, and those in which theresidue obtained at 350° C. is not more than 5% by mass are morepreferable. This is because when the residue is not more than 20% bymass, volatile residues do not disturb sintering during calcining, whichincreases the joining strength.

The anionic surfactant is preferably a carboxylic acid type containing acarboxyl group or a salt thereof, more preferably a carboxylic acid typerepresented by Formula (I) below.

R¹O(CH₂CH(R²)O)_(n1)CH₂COOR³  (I)

[In the formula, R¹ is a linear or branched alkyl group having at least7 carbon atoms, R² is any one of —H, —CH₃, —CH₂CH₃, and —CH₂CH₂CH₃, R³is —H or alkali metal, and n1 is in the range of 2 to 5.1 [036]

Furthermore, the anionic surfactant is more preferably a carboxylic acidtype represented by Formula (II) below.

R¹¹—C(O)N(R¹²)(CH₂)_(n2)COOR¹³  (II)

[In the formula, R¹¹ is a linear or branched alkyl group having at least7 carbon atoms, R¹² is any one of —H, —CH₃, —CH₂CH₃, and —CH₂CH₂CH₃, R¹³is —H, NH⁺(C₂H₄OH)₃, or alkali metal, and n2 is in the range of 1 to 5.]

Moreover, the anionic surfactant is further preferably a carboxylic acidtype represented by Formula (III) below.

R²¹—CH═CH—(CH₂)_(n3)COOR²²  (III)

[In the formula, R²¹ is a linear or branched alkyl group having at least7 carbon atoms, R²² is —H or alkali metal, and n3 is in the range of 1to 10.1

Furthermore, the anionic surfactant is further preferably a carboxylicacid type represented by Formula (IV) below.

R³¹—COOR²²  (IV)

[In the formula, R³¹ is a linear or branched alkyl group or alkoxy grouphaving at least 7 carbon atoms, which has been optionally substitutedwith OH or COOR³³ (R³³ is alkali metal), and R³² is —H or alkali metal.]

Moreover, the anionic surfactant is preferably a sulfonic acid typecontaining a sulfo group or a salt thereof, more preferably a sulfonicacid type represented by Formula (V) below.

R⁴¹—SO₃R⁴²  (V)

[In the formula, R⁴¹ is a linear or branched alkyl group, aralkyl groupor alkenyl group having at least 7 carbon atoms, which has beenoptionally substituted with OH or COOR⁴⁸ (R⁴³ is an alkyl group), or anaralkyl group, and R⁴² is —H or alkali metal.]

Furthermore, the anionic surfactant is preferably a carboxylic-sulfonicacid type containing a carboxyl group, or a salt thereof and a sulfogroup, or both salts thereof, more preferably a carboxylic-sulfonic acidtype represented by Formula (VI) below.

[In the formula, R⁵ is a linear or branched alkoxy group having at least7 carbon atoms or R⁵⁴—CONH— (R⁵⁴ is a linear or branched alkyl grouphaving at least 7 carbon atoms), R⁵² and R⁵³ are —H or alkali metal, n5is in the range of 1 to 8, n6 is in the range of 0 to 1, and n7 is inthe range of 0 to 1.]

Moreover, the anionic surfactant is preferably of a phosphate esterstructure, or a salt thereof, namely, a phosphate ester type, morepreferably a sulfate ester type represented by Formula (VII) below.

R⁶¹—O—PO(OR⁶²)OR⁶³  (VII)

[In the formula, R⁶¹ and R⁶² each are a linear or branched alkyl group,and R⁶³ is —H or alkali metal.]

The content of the surfactant is preferably up to 10% by mass withrespect to the paste. Furthermore, the content of the surfactant ispreferably not more than 2% by mass with respect to the paste, becausethis allows the surfactant to be completely volatilized by calcining.

The surfactant has preferably a contact angle of at least 10 degreeswith respect to a gold electrode with a surface roughness Ra of 0.04 μmbecause this prevents the dispersion medium contained optionally frombleeding. The contact angle of the surfactant contained in the metalpowder sintering paste can be measured with a contact angle meter.

The surfactant is preferably in a liquid state at 25° C., because thisreduces the solid content in the paste to allow a larger amount ofsilver powder to be contained, which tends to prevent voids fromoccurring.

[Organic Solvent]

The metal powder sintering paste contains preferably an organic solventas a dispersion medium. This is because uniform dispersion of the silverparticles in an organic solvent allows a high quality coating to becarried out efficiently by a method such as printing or dispensing.

There are some metal powder sintering pastes that contain resins.However, from the perspectives that sintering requires high temperatureand a resin component or the volatile portion thereof contaminatesperipheral members, the better ones are those that are free from resinand contain a volatile organic solvent as a dispersion medium. However,since the volatile organic solvent that is often used for metal powdersintering pastes has a lower viscosity than that of an adhesivecontaining resin, fine irregularities of the substrate-side electrodesurface to be joined often cause an organic solvent component towet/spread. In this case, bleeding occurs in an unexpected portion inthe substrate, which causes the organic solvent to leak onto thesubstrate bottom surface during the process or results in wire bondingdefects due to contamination. On the other hand, in the metal powdersintering paste of the present embodiment, since it contains an anionicsurfactant, even when it contains an organic solvent, the organicsolvent can be prevented from bleeding.

The dispersion medium may be one type of organic solvent or a mixture oftwo or more types of organic solvents but is preferably a mixture ofdiol and ether. This is because a metal powder sintering pastecontaining such a dispersion medium used therein can join an element anda mounting member together at a lower temperature.

The boiling point of the dispersion medium is preferably not more than300° C., more preferably 150° C. to 250° C. This is because when theboiling point of the dispersion medium is in the range of 150° C. to250° C., the change in viscosity of the metal powder sintering paste atroom temperature, which is caused by volatilization of the dispersionmedium, can be prevented and thus a good workability is obtained.Furthermore, when the dispersion medium has a boiling point in thatrange, the dispersion medium can be completely volatilized by calcining.

Examples of diol include: aliphatic diols such as ethylene glycol,1,3-propanediol, 1,4-butanediol, diethylene glycol, 1,5-pentanediol,1,6-hexanediol, dipropylene glycol, triethylene glycol, tetraethyleneglycol, 1,2-propanediol, 1,3-butanediol, 2,3-butanediol, neopentylglycol(2,2-dimethylpropane-1,3-diol), 1,2-hexanediol, 2,5-hexanediol,2-methyl-2,4-pentanediol, 3-methyl-1,3-pentanediol, and2-ethyl-1,3-hexanediol; alkylene oxide adducts of 2,2-bis(4-hydroxycyclohexyl)propane and 2,2,-bis(4-hydroxy cyclohexyl)propane; andalicyclic diols such as 1,4-cyclohexanediol and1,4-cyclohexanedimethanol.

Examples of ether include dipropylene glycol methyl ether, tripropyleneglycol methyl ether, propylene glycol n-propyl ether, dipropylene glycoln-propyl ether, propylene glycol n-butyl ether, dipropylene glycoln-butyl ether, propylene glycol phenyl ether, dipropylene glycoldimethyl ether, 1,3-dioxolane, ethylene glycol monobutyl ether,diethylene glycol monoethyl ether, diethylene glycol monobutyl ether,and ethylene glycol monoethyl ether.

When the dispersion medium is a mixture of diol and ether, the massratio of diol to ether is preferably diol:ether=7 to 9:2. This isbecause the metal powder sintering paste containing a mixture of suchorganic solvents used therein can join an element and a mounting membertogether at a lower temperature.

The content of the dispersion medium is not particularly limited sincethe required viscosity varies according to the method of applying themetal powder sintering paste. In order to control the porosity of thesintered joining layer that is obtained by calcining the metal powdersintering paste, the content of the dispersion medium is preferably upto 30% by mass with respect to the metal powder sintering paste.

[Resin]

The metal powder sintering paste is substantially free from resin.Examples of the resin include binders such as an epoxy resin and aphenolic resin as well as hardeners such as a polyamide resin.

<Method of Producing Metal Powder Sintering Paste>

The method of producing a metal powder sintering paste includes mixingan anionic surfactant with silver particles having an average particlediameter (a median diameter) of 0.3 μm to 5 μm, wherein substantially noresin is mixed together.

When the metal powder sintering paste further contains a dispersionmedium, the method of producing a metal powder sintering paste includesmixing an anionic surfactant, silver particles having an averageparticle diameter of 0.3 μm to 5 μm, and a dispersion medium, whereinsubstantially no resin is mixed together.

In the method of producing a metal powder sintering paste, mixing can becarried out at room temperature and preferably a degassing process isincluded. Inclusion of the degassing process can prevent the joiningstrength from being reduced due to gas bubbles entering under the chip.

<Metal Powder Sintering Paste>

A preferred metal powder sintering paste is one with which a conductivematerial having an electric resistance of not more than 6 μΩ·cm can beobtained by calcining in an air oven at 170° C. for 60 minutes.

A preferred metal powder sintering paste is one with which the ratio ofthe wetting/spreading diameter of the dispersion medium to the originaldiameter of the paste is not higher than 1.4, when it is applied to agold electrode with a surface roughness Ra of 0.04 μm, which then isleft to stand for 20 minutes. This is because such a metal powdersintering paste improves the leakage onto the substrate bottom surfaceand the wire bonding defects caused by contamination and thereby makesit possible to produce reliable semiconductor devices.

The metal powder sintering paste is preferably one which ischaracterized in that the ratio of the wetting/spreading diameter of thedispersion medium to the original diameter of the paste is not higherthan 3.0, when it is applied to a gold electrode with a surfaceroughness Ra of 0.48 μm, which then is left to stand for 20 minutes.This is because such a metal powder sintering paste improves the leakageonto the substrate bottom surface and the wire bonding defects caused bycontamination and thereby makes it possible to produce reliablesemiconductor devices.

<Method of Producing Conductive Material>

Furthermore, the present disclosure relates to a method of producing aconductive material, the method including a process of calcining a metalpowder sintering paste of the present disclosure.

[Calcining Conditions]

The above-mentioned calcining may be carried out, for example, in anatmosphere such as a non-oxidizing atmosphere, an air atmosphere, avacuum atmosphere, an oxygen atmosphere, a mixed gas atmosphere, or anairflow but preferably is carried out in an oxygen, ozone, or airatmosphere. This is because calcining in such an atmosphere acceleratesthe thermal diffusion of silver and thereby makes it possible to obtaina conductive material having a higher sintering strength.

In the present disclosure, the above-mentioned calcining is carried outpreferably at a temperature in the range of 150° C. to 320° C. This isbecause when calcining is carried out in this temperature range, metalcan be joined at a lower temperature than the melting point of the resinpackage on which a semiconductor element or the like is mounted.Furthermore, calcining is carried out more preferably at a temperaturein the range of 160° C. to 260° C., further preferably at a temperaturein the range of 170° C. to 195° C. This is because a lead frame, forwhich a conventional resin-containing adhesive is supposed to be used,contains a member that deteriorates at a temperature of 200° C. orhigher.

Preferably, the above-mentioned calcining is carried out so that theratio of the wetting/spreading diameter of the dispersion medium to theoriginal diameter of the paste is not higher than 1.4, when the metalpowder sintering paste is applied to a gold electrode with a surfaceroughness Ra of 0.04 μm, which then is left to stand for 20 minutes.Such a physical property can be obtained by adding the anionicsurfactant described above.

Preferably, calcining is carried out so that the ratio of thewetting/spreading diameter of the dispersion medium to the originaldiameter of the paste is not higher than 3.0, when the metal powdersintering paste is applied to a gold electrode with a surface roughnessRa of 0.48 μm, which then is left to stand for 20 minutes. Such aphysical property can be obtained by adding the anionic surfactantdescribed above.

<Conductive Material>

The conductive material of the present disclosure can be obtained bycalcining a metal powder sintering paste of the present disclosure. Theporosity of the conductive material is preferably 5% by volume to 35% byvolume, more preferably 5% by volume to 25% by volume, and furtherpreferably 5% by volume to 20% by volume. The conductive material hasthe advantage of having a high joining strength.

Preferably, the conductive material of the present disclosure has anelectrical resistance value of not higher than 50 μΩ·cm. This is becausethe lower the electric resistance thereof, the better the heatdissipation, and the less the power loss when it is used as anelectrode. The electrical resistance value is more preferably not higherthan 10 μΩ·cm and further preferably not higher than 6 μΩ·cm.

EXAMPLES

Hereinafter, with reference to examples, comparative examples, andreference examples, the metal powder sintering paste, the method ofproducing a metal powder sintering paste, the conductive material, andthe method of producing a conductive material according to the presentembodiments are described.

Example 1

Organic solvents, namely, 2-ethyl-1,3-hexanediol (0.58 g) and diethyleneglycol monobutyl ether (0.14 g), and an anionic liquid surfactant(manufactured by Sanyo Chemical Industries, Ltd., BEAULIGHT LCA-H(product name), laureth-5 carboxylic acid, liquid at 25° C., 0.09 g)were stirred with a planetary centrifugal mixer (THINKY MIXER(AWATORIRENTARO) AR-250 (product name), manufactured by THINKY) for onecycle including one minute of stirring and 15 seconds of degassing.Thus, a solvent mixture was obtained.

Flaky silver particles (manufactured by Fukuda Metal Foil & POWDER Co.,LTD., AgC-239 (product name), flake form, average particle diameter(median diameter): 2.7 μm, specific surface area: 0.7 m²/g, the contentof the particles with a particle diameter of smaller than 0.3 μm: 1% bymass, and the content of the particles with a particle diameter of notlarger than 0.5 μm: 3% by mass, 9.19 g) were weighed and then added tothe solvent mixture described above. The mixture thus obtained wasstirred with the planetary centrifugal mixer (THINKY MIXER(AWATORIRENTARO) AR-250 (product name), manufactured by THINKY) for onecycle including one minute of stirring and 15 seconds of degassing.Thus, a metal powder sintering paste was obtained (the content of thesilver particles was 91.9% by mass).

Example 2

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by Sanyo ChemicalIndustries, Ltd., BEAULIGHT LCA-25NH (product name), laureth-4carboxylic acid, liquid at 25° C., 0.09 g) was used as the liquidsurfactant.

Example 3

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by Kao Corporation,Kaosera 8110 (product name), liquid at 25° C., 0.09 g) was used as theliquid surfactant.

Example 4

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by Sanyo ChemicalIndustries, Ltd., BEAULIGHT LCA-30D (product name), sodium laureth-4carboxylate, liquid at 25° C., 0.09 g) was used as the liquidsurfactant.

Example 5

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by Sanyo ChemicalIndustries, Ltd., BEAULIGHT LCA-25N (product name), sodium laureth-4carboxylate, liquid at 25° C., 0.09 g) was used as the liquidsurfactant.

Example 6

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by Sanyo ChemicalIndustries, Ltd., BEAULIGHT ECA (product name), sodium trideceth-4carboxylate, liquid at 25° C., 0.09 g) was used as the liquidsurfactant.

Example 7

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by NOF CORPORATION,OLEOYLSARCOSINE 221P (product name), N-oleoyl-N-methylglycine (oleoylsarcosine), liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 8

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by NOF CORPORATION,NONSOUL OK-1 (product name), potassium oleate (aqueous solution), liquidat 25° C., 0.09 g) was used as the liquid surfactant.

Example 9

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by NOF CORPORATION,SOFTILT®AS-L (product name), N-dodecanoyl-N-methyl-ß-alanine sodiumsalt, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 10

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by NOF CORPORATION,SOFTILT®AT-L (product name), N-dodecanoyl-N-methyl-6-alaninetriethanolamine salt, liquid at 25° C., 0.09 g) was used as the liquidsurfactant.

Example 11

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by NOF CORPORATION,FIRET®L (product name), N-lauroyl-N-methylglycine-sodium salt, liquid at25° C., 0.09 g) was used as the liquid surfactant.

Example 12

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by NOF CORPORATION,NONSOUL OK-2 (product name), potassium oleate (aqueous solution), liquidat 25° C., 0.09 g) was used as the liquid surfactant.

Example 13

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by NOF CORPORATION,NONSOUL LK-30 (product name), potassium cocoate (aqueous solution),liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 14

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by Sanyo ChemicalIndustries, Ltd., BEAULIGHT SHAA (product name), sodium lauryl glycolcarboxylate, liquid at 25° C., 0.09 g) was used as the liquidsurfactant.

Example 15

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by Kao Corporation,LATEMUL ASK (product name), dipotassium alkenyl succinate, liquid at 25°C., 0.09 g) was used as the liquid surfactant.

Example 16

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by DKS Co. Ltd.,NEO-HITENOL S-70 (product name), disodium polyoxyethylene alkylsulfosuccinate, liquid at 25° C., 0.09 g) was used as the liquidsurfactant.

Example 17

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by Sanyo ChemicalIndustries, Ltd., BEAULIGHT ESS (product name), disodium C12-14 pareth-2sulfosuccinate, liquid at 25° C., 0.09 g) was used as the liquidsurfactant.

Example 18

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by Sanyo ChemicalIndustries, Ltd., BEAULIGHT A-5000 (product name), disodium lauramidopeg-5 sulfosuccinate, liquid at 25° C., 0.09 g) was used as the liquidsurfactant.

Example 19

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by NOF CORPORATION,PERSOFT®SK (product name), alkyl sulfate ester sodium salt, liquid at25° C., 0.09 g) was used as the liquid surfactant.

Example 20

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by NOF CORPORATION,NEWREX®R-25L (product name), linear alkylbenzene-sulfonic acid sodiumsalt, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 21

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by NOF CORPORATION,SUNBASE (product name), α-sulfofatty acid methyl-ester sodium salt,liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 22

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by NOF CORPORATION,NISSAN TRAX®K-40 (product name), polyoxyethylene lauryl ethersulfate-sodium salt, liquid at 25° C., 0.09 g) was used as the liquidsurfactant.

Example 23

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by DKS Co. Ltd., NEOGENAO-90 (product name), sodium α-olefin sulfonate, powder at 25° C., 0.09g) was used as the liquid surfactant.

Example 24

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by Kao Corporation,LATEMUL PS (product name), sodium alkane sulfonate, liquid at 25° C.,0.09 g) was used as the liquid surfactant.

Example 25

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat an anionic liquid surfactant (manufactured by DKS Co. Ltd., PLYSURFDBS (product name), sodium alkyl phosphate, liquid at 25° C., 0.09 g)was used as the liquid surfactant.

Comparative Example 1

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat a cationic liquid surfactant (manufactured by Sanyo ChemicalIndustries, Ltd., LEBON TM-16 (product name), cetrimonium chloride,liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Comparative Example 2

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat a nonionic liquid surfactant (manufactured by Sanyo ChemicalIndustries, Ltd., EMULMIN NL-70 (product name), laureth-7, liquid at 25°C., 0.09 g) was used as the liquid surfactant.

Comparative Example 3

A metal powder sintering paste (the content of the silver particles was91.9% by mass) was obtained in the same manner as in Example 1 exceptthat a nonionic liquid surfactant (manufactured by Kao Corporation,Kaosera 8200 (product name), liquid at 25° C., 0.09 g) was used as theliquid surfactant.

Comparative Example 4

Organic solvents, namely, 2-ethyl-1,3-hexanediol (0.65 g) and diethyleneglycol monobutyl ether (0.16 g) were mixed together and then stirredwith the planetary centrifugal mixer (THINKY MIXER (AWATORIRENTARO)AR-250 (product name), manufactured by THINKY) for one cycle includingone minute of stirring and 15 seconds of degassing. Thus, a solventmixture was obtained.

Flaky silver particles (manufactured by Fukuda Metal Foil & POWDER Co.,LTD., AgC-239 (product name), 9.19 g) were weighed and then added to thesolvent mixture. The mixture thus obtained was stirred with theplanetary centrifugal mixer (THINKY MIXER (AWATORIRENTARO) AR-250(product name), manufactured by THINKY) for one cycle including oneminute of stirring and 15 seconds of degassing. Thus, a metal powdersintering paste was obtained (the content of the silver particles was91.9% by mass).

Tables 1 and 2 indicate the results of the measurements of the bleedratio, electric resistance, and joining strength of Examples (Exs.) 1 to25 and Comparative Examples (Comp. Exs.) 1 to 4. Since those with anelectric resistance of 10 μΩ·cm or higher, which was obtained as theresult of the measurement of the electric resistance, cannot be expectedto have a high joining property, they were partly excluded from theevaluation of the joining strength. Furthermore, Table 3 indicates theresults of the measurements of the electric resistance that was measuredwith various calcining temperatures with respect to Example 1 thatshowed a good result in Table 1 and Comparative Example 4 that containedno surfactant added thereto. The measurements of the bleed ratio,electric resistance, and joining strength were carried out as follows.

Each metal powder sintering paste obtained as described above wasapplied onto a substrate having a gold electrode with a surfaceroughness Ra of 0.04 μm on its surface by a stamping method to have adiameter of 170±50 μm, which then was left to stand for 20 minutes.Thereafter, the diameter including the dispersion medium that had bledwas measured, and then the bleed ratio was calculated as the ratio ofthe diameter after bleeding to the original diameter of the paste.

Each metal powder sintering paste obtained as described above wasapplied to a glass substrate (thickness: 1 mm) by the screen printingmethod to have a thickness of 100 μm. Each glass substrate with aconductive material composition applied thereto was heated in an airatmosphere for 60 minutes at 170° C. in the case of Tables 1 and 2 andat specified temperatures indicated in Table 3. The electric resistanceof each wiring (a conductive material) thus obtained was measured withMCP-T600 (product name) (manufactured by Mitsubishi ChemicalCorporation) by a four-terminal method.

Each metal powder sintering paste obtained as described above wasapplied onto a substrate having a silver electrode on its surface by thestamping method, and a semiconductor element that has a silver electrodeon its reverse side and a sapphire substrate with an outer dimension of500×300 μm and a thickness of 150 μm was mounted thereon. The substratehaving the semiconductor element mounted thereon with the metal powdersintering paste interposed therebetween was heated with an air oven at170° C. for 60 minutes and thereafter, it was cooled. Then, shear forcewas applied in the direction of removing the semiconductor element fromthe substrate and the strength at which the semiconductor element wasdetached was measured as the joining strength.

Moreover, surfactants used in Examples 1 to 15 and Comparative Examples1 to 3 were subjected to volatility evaluation by TG-DTA, and theresidue (% by mass) at 350° C. was additionally recorded in Tables 1 and2. The TG-DTA was carried out using TG/DTA6300 manufactured by SIINanoTechnology Inc. and an aluminum open sample pan under the conditionsincluding a sample mass of 5 mg, an air flow rate of 200 mL/min, aninitial temperature of 30° C., and a temperature rising rate of 2°C./min, and then the mass of the residue obtained when the temperaturereached 350° C. was calculated as a ratio thereof to the initial mass.

TABLE 1 Surfactant Properties Paste Properties Residue Electric DieShear at 350° C. Bleed Resistance Strength Polarity Type PrincipalComponent Rational Formula (wt %) Ratio (μΩ · cm) (MPa) Ex. 1 AnionicCarboxylic acid type C₁₂H₂₅O(CH₂CH₂O)_(n)CH₂COOH (n = 4) 2.2 1.55 4.2844 Ex. 2 Anionic Carboxylic acid type C₁₂H₂₅O(CH₂CH₂O)_(n)CH₂COOH (n =2.5) 2.2 1.27 4.08 48 Ex. 3 Anionic Carboxylic acid typeC₁₂H₂₅O(CH₂CH₂O)_(n)CH₂COOH (n = 4.5) 3.3 1.49 4.53 47 Ex. 4 AnionicCarboxylic acid type C₁₂H₂₅O(CH₂CH₂O)_(n)CH₂COONa (n ≠ 3) 5.5 1.45 5.4322 Ex. 5 Anionic Carboxylic acid type C₁₂H₂₅O(CH₂CH₂O)_(n)CH₂COONa (n ≠3) 5.2 1.41 5.40 17 Ex. 6 Anionic Carboxylic acid typeC₁₃H₂₇O(CH₂CH₂O)_(n)CH₂COONa (n ≠ 3) 21.6 1.43 7.07 9 Ex. 7 AnionicCarboxylic acid type C₁₇H₃₃C(O)N(CH₃)CH₂COOH 17.2 1.44 5.49 26 Ex. 8Anionic Carboxylic acid type C₈H₁₇CH═CHC₇H₁₄COOK 7.5 1.48 5.66 31 Ex. 9Anionic Carboxylic acid type [R—C(O)N(CH₃)C₂H₄COO]⁻Na⁺ 8.2 1.43 5.65 22Ex. 10 Anionic Carboxylic acid type [R—C(O)N(CH₃)C₂H₄COO]⁻NH⁺(C₂H₄OH)₃5.3 1.38 4.23 35 Ex. 11 Anionic Carboxylic acid type[C₁₁H₂₃—C(O)N(CH₃)CH₂COO]⁻Na⁺ 8.9 1.37 5.42 22 Ex. 12 Anionic Carboxylicacid type CH₃(CH₂)₇CH═CH(CH₂)₇COOK 12.7 1.47 6.41 15 Ex. 13 AnionicCarboxylic acid type R—COOK 16.7 1.43 6.74 17 Ex. 14 Anionic Carboxylicacid type C₁₀H₂₁CH(OH)CH₂OCH₂COONa 12.0 1.46 5.82 14 Ex. 15 AnionicCarboxylic acid type R—CH(COOK)CH₂COOK 28.0 1.48 5.84 10

TABLE 2 Surfactant Properties Paste Properties Residue Electric DieShear at 350° C. Bleed Resistance Strength Polarity Type PrincipalComponent Rational Formula (wt %) Ratio (μΩ · cm) (MPa) Ex. 16 AnionicCarboxylic-sulfonic RO(CH₂CH₂O)_(n)COCH₂CH(SO₃Na)—COONa(n ≠ 7) — 1.424.72 18 acid type Ex. 17 Anionic Carboxylic-sulfonicRO(CH₂CH₂O)_(n)COCH(SO₃Na)—CH₂COONa(n ≠ 2) — 1.45 15.87 — acid type Ex.18 Anionic Carboxylic-sulfonic C₁₁H₂₃CONHCH₂CH₂O(CH₂CH₂O)_(n)COCH — 1.475.19 13 acid type (SO₃Na)—CH—COONa(n ≠ 5) Ex. 19 Anionic Sulfonic acidtype ROSO₃Na — 1.52 4.63 13 Ex. 20 Anionic Sulfonic acid typeC_(n)H_(2n+1)(C₆H₄)SO₃Na(n = 10−16) — 1.49 6.05 15 Ex. 21 AnionicSulfonic acid type R—CH(—SO₃Na)—COOCH₃ — 1.49 42.12 — Ex. 22 AnionicSulfonic acid type C₁₂H₂₅O(CH₂CH₂O)_(n)SO₃Na — 1.53 26.75  1 Ex. 23Anionic Sulfonic acid type R—CH═CH—CH₂—SO₃Na — 1.49 35.26 — Ex. 24Anionic Sulfonic acid type R—SO₃Na — 1.36 15.03 — Ex. 25 AnionicPhosphorus acid type ROPO(—OR)ONa — 1.38 7.37  9 Comp. Cationic —[C₁₆H₃₃—N(CH₃)₃]⁺Cl⁻ 1.4 1.70 56.54  5 Ex. 1 Comp. Nonionic —C₁₂H₂₅O(CH₂CH₂O)_(n)H (n ≠ 7) 0.3 2.29 5.36 32 Ex. 2 Comp. Nonionic —Unsaturated carboxylic acid ester 43.0  2.20 45.57 10 Ex. 3 Comp. None —— — 1.72 4.53 14 Ex. 4

TABLE 3 Calcining Temperature 172° C. 177° C. 182° C. 185° C. 192° C.Comp. No surfactant 5.2 μΩ · cm 5.0 μΩ · cm 4.5 μΩ · cm 4.2 μΩ · cm 3.9μΩ · cm Ex. 4 Ex. 1 BEAULIGHT LCA-H 4.5 μΩ · cm 4.3 μΩ · cm 3.8 μΩ · cm3.8 μΩ · cm 3.9 μΩ · cm

As indicated in Tables 1 and 2, as compared to Comparative Example 4, inwhich no surfactant was added, in Examples 1 to 25, in each of which ananionic surfactant was added, the bleed ratio was decreased and thus animprovement was made. On the other hand, in Comparative Example 1, inwhich a cationic surfactant was added, no change was observed, and inComparative Examples 2 and 3, in each of which a nonionic surfactant wasadded, the bleed ratio adversely increased and was worsened. Thus, it isclear that an anionic surfactant improves the bleed resistance.

On the other hand, with respect to sinterability, among Examples 1 to15, in each of which the surfactant was an anionic carboxylic acid type,only Examples 6 and 15 that had poor volatility with the residue at 350°C. exceeding 20% by mass had reduced sinterability as compared toComparative Example 4, in which no surfactant was added. Furthermore,high joining strength exceeding 40 MPa was obtained only in Examples 1to 3 having good volatility with the residue at 350° C. being not morethan 5% by mass.

On the other hand, in Comparative Example 1, in which a cationicsurfactant was used, the residue at 350° C. was not higher than 5% bymass but the joining strength was low, specifically 5 MPa. Among thosein which a nonionic surfactant was used, Comparative Example 3 with poorvolatility had a low joining strength, but Comparative Example 2 withhigh volatility had a higher joining strength. However, as compared toExamples 1 to 3, in each of which an anionic surfactant was used, theyhad better volatility but the joining strength thereof did not reach 40MPa.

The results described above indicate that the use of an anionicsurfactant improves the bleed resistance, and the use of a surfactantwith high volatility can maintain or improve the joining strength.Specifically, the residue at 350° C. is preferably not more than 20% bymass, most preferably not more than 5% by mass. In Example 1, in whichan improvement was observed, it was confirmed that a low resistance wasachieved at lower temperature as indicated in Table 3, and it isconsidered to be sufficient that the calcining temperature is around180° C., which is equivalent to that of a resin adhesive material.

Example 26

Organic solvents, namely, 2-ethyl-1,3-hexanediol (0.38 g) and diethyleneglycol monobutyl ether (0.09 g), and an anionic liquid surfactant(manufactured by Sanyo Chemical Industries, Ltd., BEAULIGHT LCA-H(product name), laureth-5 carboxylic acid, liquid at 25° C., 0.05 g)were stirred with the planetary centrifugal mixer (THINKY MIXER(AWATORIRENTARO) AR-250 (product name), manufactured by THINKY) for onecycle including one minute of stirring and 15 seconds of degassing.Thus, a solvent mixture was obtained.

Flaky silver particles (manufactured by Fukuda Metal Foil & POWDER Co.,LTD., AgC-239 (product name), flake form, average particle diameter(median diameter): 2.5 μm, specific surface area: 0.7 m²/g, the contentof the particles with a particle diameter of smaller than 0.3 μm: 1% bymass, and the content of the particles with a particle diameter of notlarger than 0.5 μm: 3% by mass, 5.00 g) were weighed and then added tothe solvent mixture described above. The mixture thus obtained wasstirred with the planetary centrifugal mixer (THINKY MIXER(AWATORIRENTARO) AR-250 (product name), manufactured by THINKY) for onecycle including one minute of stirring and 15 seconds of degassing.After stirring, a mesh (330T mesh, wire diameter: 40 μm) was used tofiltrate it. Thus, a metal powder sintering paste was obtained (thecontent of the silver particles was 90.6% by mass).

Example 271

A metal powder sintering paste (the content of the silver particles was90.6% by mass) was obtained in the same manner as in Example 26 exceptthat an anionic liquid surfactant (manufactured by Sanyo ChemicalIndustries, Ltd., BEAULIGHT LCA-25NH (product name), laureth-4carboxylic acid, liquid at 25° C., 0.05 g) was used as the liquidsurfactant.

Example 281

A metal powder sintering paste (the content of the silver particles was90.6% by mass) was obtained in the same manner as in Example 26 exceptthat an anionic liquid surfactant (manufactured by Kao Corporation,Kaosera 8110 (product name), liquid at 25° C., 0.05 g) was used as theliquid surfactant.

Comparative Example 51

A metal powder sintering paste (the content of the silver particles was90.6% by mass) was obtained in the same manner as in Example 26 exceptthat a nonionic liquid surfactant (manufactured by Kao Corporation,Kaosera 8200 (product name), liquid at 25° C., 0.05 g) was used as theliquid surfactant.

Comparative Example 6

Organic solvents, namely, 2-ethyl-1,3-hexanediol (3.01 g) and diethyleneglycol monobutyl ether (0.75 g) were mixed together and then stirredwith the planetary centrifugal mixer (THINKY MIXER (AWATORIRENTARO)AR-250 (product name), manufactured by THINKY) for one cycle includingone minute of stirring and 15 seconds of degassing. Thus, a solventmixture was obtained.

Flaky silver particles (manufactured by Fukuda Metal Foil & POWDER Co.,LTD., AgC-239 (product name), 40.00 g) were weighed and then added tothe solvent mixture. The mixture thus obtained was stirred with theplanetary centrifugal mixer (THINKY MIXER (AWATORIRENTARO) AR-250(product name), manufactured by THINKY) for one cycle including oneminute of stirring and 15 seconds of degassing. After stirring, a mesh(330T mesh, wire diameter: 40 μm) was used to filtrate it. Thus, a metalpowder sintering paste was obtained (the content of the silver particleswas 91.4% by mass).

Table 4 indicates the results of the measurements of the contact angleof the raw material surfactant with respect to the gold electrode, thebleed ratio of the paste itself, and the joining strength of Examples 26to 28 and Comparative Examples 5 and 6. The contact angle of the rawmaterial surfactant with respect to the gold electrode, the bleed ratio,and the joining strength were measured as follows.

With respect to each surfactant itself used in Examples 26 to 28 andComparative Example 5, the contact angle with respect to the goldelectrode with a surface roughness Ra of 0.04 μm was measured using acontact angle meter CA-X150.

Each metal powder sintering paste obtained as described above wasapplied onto a substrate having a gold electrode on its surface by thestamping method, and a semiconductor element that has a silver electrodeon its reverse side and a sapphire substrate with an outer dimension of500×300 μm and a thickness of 150 μm was mounted thereon. The substratehaving the semiconductor element mounted thereon with the metal powdersintering paste interposed therebetween was heated with an air oven at175° C. for 90 minutes and thereafter, it was cooled. Then, shear forcewas applied in the direction of removing the semiconductor element fromthe substrate and the strength at which the semiconductor element wasdetached was measured as the joining strength.

Each metal powder sintering paste obtained as described above wasapplied onto two types of substrates, each of which has a gold electrodeon its surface and is different in surface roughness, by the stampingmethod to have a diameter of 170±50 μm, which then was left to stand for20 minutes. Thereafter, the diameter including the dispersion mediumthat had bled was measured, and then the bleed ratio was calculated asthe ratio of the diameter after bleeding to the original diameter of thepaste.

TABLE 4 Ex. 26 Ex. 27 Ex. 28 Comp. Ex. 5 Comp. Ex. 6 Surfactant NameBEAULIGHT BEAULIGHT Kaosera Kaosera No Physical LCA-H LCA-25NH 8110 8200Surfactant Property Polarity Anion Anion Anion Nonion — Contact Angle13.3-14.7 10.6-13.5 10.2-13.3 6.6-8.4 — (degree) with respect to GoldElectrode with Ra 0.04 μm Joining Strength (MPa) 36 27 21 9 38 BleedGold Electrode 1.4 1.2 1.4 2.2 1.5 Ratio with Ra 0.04 μm Gold Electrode2.6 1.7 3.0 3.7 3.3 with Ra 0.48 μm

As indicated in Table 4, as compared to Comparative Example 6, in whichno surfactant was used, in Examples 26 to 28, in each of which ananionic surfactant with a contact angle of larger than 10 degrees withrespect to the gold electrode with RA 0.04 μm was used, the bleed ratiowas reduced in both cases of the gold electrode with Ra 0.04 μm and thegold electrode with Ra 0.48 μm and thus, it was confirmed that bleedingtends not to occur. Furthermore, under all conditions, the higher theroughness Ra of the gold electrode, the higher the bleed ratio. InComparative Example 5, in which a nonionic surfactant with a contactangle of smaller than 10 degrees with respect to the gold electrode withRa 0.04 μm was used, the bleed ratio increased and thus worse resultswere obtained. These results showed that bleeding can be prevented byadding a surfactant with a larger contact angle with respect to theelectrode.

Moreover, with respect to the joining strength, a sufficiently highjoining strength exceeding 20 MPa was obtained in Examples 26 to 28 ascompared to Comparative Example 6, in which no surfactant was added.

The metal powder sintering pastes of the present embodiments can beused, for example, for producing heat-resistant power wiring, componentelectrodes, die attaches, microbumps, flat panels, solar wiring, etc.,for wafer bonding, and for producing electronic components that areproduced using these in combination. Furthermore, the method ofproducing a conductive material of the present embodiment also can beused, for example, for producing a light-emitting device in which alight-emitting element such as LED or LD is used.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

1. A metal powder sintering paste, the paste comprising, as a principalcomponent, silver particles having a median diameter of 0.3 μm to 5 μm,and further comprising an anionic surfactant, and being substantiallyfree from resin, wherein a content of the surfactant is not more than 2%by mass with respect to the paste, and wherein a bleed ratio is lessthan 1.70, wherein the bleed ratio is calculated as a ratio of adiameter after bleeding to an original diameter of the paste when thepaste is applied onto a substrate having a gold electrode with a surfaceroughness Ra of 0.04 μm on its surface by a stamping method to have adiameter of 170±50 μm, which then is left to stand for 20 minutes, andthe diameter after bleeding is measured by measuring the diameterincluding a dispersion medium that had bled.
 2. The metal powdersintering paste according to claim 1, wherein the surfactant is one inwhich when the temperature is increased withthermogravimetric-differential thermal analysis (TG-DTA) from roomtemperature to 350° C. at 2° C./min, a residue is reduced to not morethan 20% by mass with respect to the initial mass.
 3. The metal powdersintering paste according to claim 1, wherein the surfactant is asaturated carboxylic acid represented by Formula (I) below:R¹O(CH₂CH(R²)O)_(n1)CH₂COOR³  (I) wherein R¹ is a linear or branchedalkyl group having at least 7 carbon atoms, R² is any one of —H, —CH₃,—CH₂CH₃, and —CH₂CH₂CH₃, R³ is —H or alkali metal, and n1 is in a rangeof 2 to
 5. 4. The metal powder sintering paste according to claim 1,wherein the surfactant is liquid at 25° C.
 5. The metal powder sinteringpaste according to claim 1, wherein the silver particles are in a flakeform.
 6. The metal powder sintering paste according to claim 1, whereinin the silver particles, a content of particles with a particle diameterof smaller than 0.3 μm is not more than 5% by mass.
 7. The metal powdersintering paste according to claim 6, wherein in the silver particles, acontent of particles with a particle diameter of smaller than 0.5 μm isnot more than 15% by mass.
 8. The metal powder sintering paste accordingto claim 1, wherein the metal powder sintering paste further comprisesan organic solvent as a dispersion medium.
 9. The metal powder sinteringpaste according to claim 8, wherein the organic solvent has a boilingpoint in a range of 150 to 250° C.
 10. The metal powder sintering pasteaccording to claim 1, capable of being sintered to form a conductivematerial having an electric resistance of not higher than 6 μΩ·cm. 11.The metal powder sintering paste according to claim 1, wherein a contentof the silver particles is at least 70% by mass with respect to thepaste.
 12. A method of producing a conductive material, comprising astep of calcining a metal powder sintering paste, wherein the metalpowder sintering paste comprises, as a principal component, silverparticles having a median diameter of 0.3 μm to 5 μm, and an anionicsurfactant, and is substantially free from resin, wherein the surfactantis saturated carboxylic acid represented by Formula (I) below:R¹O(CH₂CH(R²)O)_(n1)CH₂COOH  (I) wherein R¹ is a linear or branchedalkyl group having at least 7 carbon atoms, R² is any one of —H, —CH₃,—CH₂CH₃, and —CH₂CH₂CH₃, and n1 is in a rage of 2 to 5, and wherein thecontent of the silver particles is at least 70% by mass with respect tothe paste.
 13. The method of producing a conductive material accordingto claim 12, wherein the calcining is carried out at a temperature in arange of 160° C. to 300° C.
 14. The method of producing a conductivematerial according to claim 12, wherein the calcining is carried out inan air oven at 160° C. to 250° C. for 30 to 120 minutes.
 15. The methodof producing a conductive material according to claim 12, wherein thesurfactant is one in which when the temperature is increased with TG-DTAfrom room temperature to 350° C. at 2° C./min, the residue is reduced tonot more than 20% by mass with respect to the initial mass.
 16. Themethod of producing a conductive material according to claim 12, thecontent of the surfactant is not more than 2% by mass with respect tothe paste.
 17. The method of producing a conductive material accordingto claim 12, wherein in the silver particles, the content of particleswith a particle diameter of smaller than 0.3 μm is not more than 5% bymass.
 18. The method of producing a conductive material according toclaim 12, the metal powder sintering paste further comprises an organicsolvent as a dispersion medium and the organic solvent has a boilingpoint in a range of 150 to 250° C.
 19. The method of producing aconductive material according to claim 12, wherein a conductive materialobtained by calcining the metal powder sintering paste has an electricresistance of not higher than 6 μΩ·cm.
 20. The method of producing aconductive material according to claim 12, the metal powder sinteringpaste is one with which the ratio of the wetting/spreading diameter ofthe dispersion medium to the original diameter of the paste is nothigher than 1.4, when it is applied to a gold electrode with a surfaceroughness Ra of 0.04 μm, which then is left to stand for 20 minutes. 21.The method of producing a conductive material according to claim 12, themetal powder sintering paste is one with which the ratio of thewetting/spreading diameter of the dispersion medium to the originaldiameter of the paste is not higher than 3.0, when it is applied to agold electrode with a surface roughness Ra of 0.48 μm, which then isleft to stand for 20 minutes.